CORRELATED RESPONSES TO SELECTION FOR HIGH OR LOW INCIDENCE OF TIBIALDYSCHONDROPLASIA IN BROILERS

Selection for a high or a low incidence of tibial dyschondroplasia (TD ) at 7 wk of age was practiced for four generations in broiler breeder s keeping an unselected control (C) line and using low intensity X-ray imaging for TD diagnosis. The incidence of TD, BW at 4 and 7 wk of ag e, age at first egg, and egg production for the first 12 wk of product ion were determined in the four generations. Age at initial semen prod uction and semen volume and concentration of males were determined onl y in Generations 3 and 4. The TD scores and BW at 7 wk were collected on 1,366, 1,301, and 1,389 broilers from the C, high (H), and low (L) incidence of TD lines. Incidence of TD at 7 wk in the fourth generatio n was 16.1, 61.9, and 3.5% for the C, H, and L lines, respectively. Th ere were no significant differences in BW between the H and L lines in Generations 1, 3, or 4. However, in Generation 2, the H line was heav ier (P <.01) than the L line. Second generation H line broilers with T D were heavier than those without TD at 4 and 7 wk of age (P < .01). T here were no significant differences in age at first egg, age at semen production, or in semen volume between the H and the L lines. Egg pro duction in the L line was greater (P <.01) than that of the H line in all four generations, and semen concentration was also greater (P <.01 ) in Generations 3 and 4. Correlated responses to selection in the H a nd the L line were estimated per generation and per weighted selection differential (WCSD) for the traits that were recorded in the four gen erations. Regression coefficients of cumulative response per generatio n and per unit of WCSD were not significant.